"Absolutely," he said. "I'm ready to get out there, get my feet wet again."
He went 1-for-4 with a double in his return.
"To get through Spring Training when you still have trepidation about an injury and get out on the field for a regular-season game, it's got to be rewarding for him, for a guy that put himself in this position again," manager Bob Melvin said.
Sizemore, 28, learned third base for the A's upon his move from Detroit in 2011, and that's where big things were expected from him in 2012, before he went down to injury on the first day of spring workouts last year.
Since, the A's have found a stable third-base option in Josh Donaldson, and they've asked Sizemore to return to his natural position at second base, where he won a job out of Spring Training -- for the first time since 2010 -- alongside platoon partner Eric Sogard.
"I was definitely relieved to make the team, obviously, and I'm just excited and happy and all of the things that come with being back on the field," he said. "It's been awhile since I broke with the team. I'm just hoping to make the most of my opportunity."
Jane Lee is a reporter for MLB.com. Read her blog, Major Lee-ague, and follow her on Twitter @JaneMLB. This story was not subject to the approval of Major League Baseball or its clubs.